1. Field of the Invention
This invention relates to an engine vibration sensor, more particularly to an engine vibration sensor which comprises a vibrator set to resonate at a predetermined, specific frequency of vibration of the engine to which it is attached, the vibration of the vibrator being converted into an electric signal representing the engine vibration.
2. Description of the Prior Art
Engines play an important role in ships, automobiles and many other devices and machines that contribute greatly to the quality of modern life. However, unless an engine is used under optimum operating conditions it is apt to suffer a decline in power output, abnormal vibration, abnormal wear and/or a decrease in fuel efficiency.
In order to assure that an engine is operating under optimum conditions, it is necessary to accurately monitor its actual operating state. One device known to be highly useful for this purpose is the engine vibration sensor. The usefulness of this device derives from the fact that the vibration of an engine at specific frequencies accurately reflects the operating state of the engine so that by measuring the magnitude and characteristics of the vibration at specific frequencies there can be obtained a considerable amount of data for use in optimizing the engine operating conditions. The specific frequency or frequencies selected for monitoring depend on the actual operating state of the engine. In the following, a specific example will be described in connection with the knocking vibration frequency of an engine.
Ordinarily, when ignition occurs too early in an engine, knocking and decreased power result. A decrease in power also occurs when the ignition is too late. It is therefore necessary to optimize the ignition advance so as to obtain maximum power and operating efficiency without causing the engine to knock. It is, however, no easy matter to determine the optimum value beforehand since it is dependent on the type of engine, the specific character of the engine, the number of revolutions and the intake pressure. Conventionally, the ignition advance has been set mechanically or electrically on the basis of the engine speed and the intake pressure. This method does not, however, always result in the optimum ignition advance. What has been done in actual practice, therefore, was to set the advance angle slightly smaller than the optimum value in order to prevent knocking, even though this meant that it was impossible to realize maximum power.
This expedient is no longer satisfactory since it runs counter to current demands for better engine fuel economy and fuel efficiency. The need for optimizing ignition advance is particularly strong in the case of the turbocharged engines that are being developed specifically for the purpose of reducing fuel consumption and boosting power. To meet the requirements of these engines there has been developed a knock control system for obtaining maximum efficiency wherein the ignition advance is automatically controlled using trace knocking as an index. For this system to operate effectively it is necessary to be able to measure the trace knocking of the engine quickly and accurately. Several types of engine vibration sensors have been developed and used for this purpose.
These known sensors include magnetostrictive, piezoelectric disk, piezoelectric cantilever and various other types, but none has been able to provide the required performance. For example, some are capable of precise measurement only within a limited temperature range while others can provide reliable results only at specific engine rotational speeds or under other specific measurement conditions. As a result, it has been difficult to carry out reliable engine knock control. Because of this, there has been desired a vibration sensor capable of reliably distinguishing between vibration peculiar to knocking and other miscellaneous types of vibration regardless of changes in the engine speed, temperature and other measurement conditions. Also, as most engines are of the multi-cylinder type, there has been desired a sensor capable of measuring the knocking vibration at a number of cylinders so as to make it possible to use a single sensor for optimizing the ignition advance for all cylinders of a multi-cylinder engine.
FIG. 1 shows engine vibration waveforms obtained by attaching a non-resonating vibrator having flat frequency characteristics to an engine and converting the engine vibration obtained through this vibrator to an electrical signal. The waveform shown in FIG. 1A is that obtained for an engine operating under a normal state of combustion without knocking while that shown in FIG. 1B is that obtained for an engine operating under an abnormal state of combustion with knocking. It will be noted from these graphs that in both cases the vibration wave periodically grows large in amplitude in synchronization with the combustion timing. In the case of FIG. 1B showing the waveform for an abnormal state of combustion, however, in addition to the periodic large amplitude waves there can be seen large amplitude vibration waves at positions somewhat shifted from the timing of the vibration peaks.
FIG. 2 shows the frequency spectra for the waveforms shown in FIGS. 1A and 1B. FIG. 2A shows the frequency spectrum for an engine operating under a normal state of combustion without knocking and FIG. 2B shows the frequency spectrum for an engine operating under an abnormal state of combustion with knocking. As is clear from these two graphs, the frequency spectrum in the case of normal combustion with no knocking is flat while that in the case of abnormal combustion with knocking is characterized by the occurrence of peaks at a specific frequency region.
Thus the vibration generated by knocking occurs within a frequency range extending from about 6 to 8 KHz while general vibration not related to knocking is spread over a wide range of frequencies. Because of this, by using a vibration sensor provided with a vibrator having resonant frequency characteristics which, as shown in FIG. 3, are coincident with the knocking vibration frequencies, it is possible to measure the engine's knocking vibration independently of its other general vibration.
In the case of the conventional vibration sensors used heretofore, however, it is often difficult to make the resonant frequency of the vibrator coincide with the knocking vibration frequency and under some measurement conditions the resonant frequency of the vibration sensor will be different from the knocking vibration frequency. When the two frequencies fail to coincide, the sensor becomes incapable of distinguishing between the general vibration arising from various parts of the engine and the vibration peculiar to knocking, meaning that the measurement characteristics of the sensor will be degraded.
The causes behind this degradation in measurement characteristics can be divided into those arising from the vibration sensor and those arising from the engine.
The first cause that can be mentioned in conjunction with the vibration sensor is poor quality, meaning poor quality of the sensor itself or of its state of attachment to the engine. More specifically, the resonant frequency of a vibration sensor is strongly affected by the material, dimensions and state of attachment of the sensor. Using current fabrication processes, it is not possible, no matter how much care is taken, to produce vibration sensors having any less scatter among their resonant frequencies than about 1 KHz. This means that many of the sensors will have resonant frequencies which are not in good coincidence with engine knocking vibration frequency and will thus have poor characteristics
A second cause is that the resonant frequency of a vibration sensor varies with temperature. More specifically, the Young's modulus and the state of attachment of the vibrator of a vibration sensor changes with temperature and changes in these factors in turn cause changes in the resonant frequency. Since engines are commonly used over a wide range of temperatures extending from around -30 to around +120.degree. C., vibration in resonant frequency because of temperature change is a major cause for degradation of the characteristics of a vibration sensor.
A third cause is the deterioration of vibration sensor characteristics that occurs with use. More specifically, even if the resonant frequency of a vibration sensor is adjusted to the knocking vibration frequency of the engine at the time it is attached to the engine, the state of attachment of its vibrator will deteriorate with use, with a resulting change in the resonant frequency. The consequent deviation between the resonant frequency and the engine knocking vibration frequency is another important cause of degradation of vibration sensor characteristics.
Next a look will be taken at factors on the side of the engine which can lead to a discrepancy arising between the resonant frequency of a vibration sensor and the knocking vibration frequency of the engine to which it is attached. First there are numerous operating conditions of the engine which cause variation in the engine's knocking vibration frequency such as the engine temperature, intake air pressure, and speed of rotation. The knocking vibration frequency will also vary from engine to engine even among engines of the same type. What is more, it will vary from cylinder to cylinder in one and the same engine. The resulting difference between the resonant frequency of the vibration sensor and the knocking vibration frequency of the engine to which it is attached constitutes still another major cause for degradation of the characteristics of the vibration sensor.
The conventional vibration sensor is further disadvantageous in that it is not capable of accurately measuring the knocking vibration at a plurality of cylinders each of which has a different knocking vibration frequency as is invariably the case in an actual engine. Therefore, in the conventional knock control system using trace knocking as an index, it has been the practice to set the angle of ignition advance for the cylinders not fitted with vibration sensors at a slightly smaller angle than the cylinder fitted with the vibration sensor. Consequently, knocking at the cylinders not fitted with vibration sensors can only indirectly be prevented by controlling the angle of ignition advance for the cylinder provided with the vibration sensor using the trace knocking thereof as an index. This means that in the knock control systems using conventional vibration sensors, the effect of the system could not be fully extended to all of the engine's cylinders.